Dark Matter and Cosmic Web Story

by Jaan Einasto

  After the Tallinn symposium in 1977 I understood that for cosmological studies an observatory in a very good climatical region is needed. To maintain such an observatory the collective efforts of many scientific centres are needed. Examples of such joint observatories are the European Southern Observatory, the National Optical Astronomy Observatory in the USA and similar observatories in Japan, China and elsewhere.

  To investigate the possibility of the creation of a joint astronomical observatory in the USSR I visited in autumn 1977 Byurakan Observatory and discussed the idea with Viktor Ambartsumian, the most influential astronomer of the USSR. We both agreed that the creation of a joint astrophysical observatory would be very important for the further development of astronomy. Ambartsumian wrote a letter to the vice-president of the USSR Academy of Sciences with the suggestion to consider the possibility of creating a joint astrophysical observatory. With this letter all doors were open to start actual preparations.

  Next year we started conducting expeditions to Central Asia to find a possible location for the joint observatory, because observing conditions within the Soviet Union were the best in the desert mountain regions of Central Asia. First I visited with some of my collaborators all Central Asian republics of the USSR, where local academies or universities already had their observatories. Soon we understood that the best location is in the Maidanak mountain region of the Uzbek SSR, situated a few hundred kilometers south of Samarkand. Here several observatories already had their Southern bases: the Moscow and the Leningrad universities and the Lithuanian Academy of Sciences. The major advantage of the Maidanak mountain was the availability of roads to the mountain, built for a military observatory a few kilometers away from the university observatories.

  It soon became evident that it is not easy to create the joint observatory, because everyone wanted to have its own observatory. Thus we soon changed our initial plans and started to prepare our own Southern observational base. Our preparations to erect a Southern base were otherwise successful, but financing the construction of the telescope was a weak point. Finally we found a suitable source: in the late 1980’s it was possible to get a subsidy (similar to grants at present time) from the central sources in Moscow. By explaining our request with the need to study the structure of the Universe on large scales, we received subsidies for several years. We spent them on the engineering of the telescope in the laboratory of the Leningrad University; we also bought a 2.3-m diameter mirror blank for the telescope, which was perfect for our needs.

  During the preparations to build our Southern observational base I often visited Byurakan Observatory to discuss with Viktor Ambartsumian the progress of the project. One of my last visits was after the Armenian earthquake in 1988. When scientific discussions were over, we started to speak on other interesting events, and Ambartsumian told me an anecdote. During the earthquake many people were buried under beton-blocks used in building standard houses at the time. When rescue workers found one man alive under the block, his first question was: “is Armenia already free”. When he heard the answer that not yet, he asked to put him again under the block and take out when the country is free. It was the time when Soviet republics had started their independence movements, and Ambartsumian was one of the leaders of the Armenian freedom movement.

  Fig. 5.11 Discussing with Viktor Ambartsumian the development of astronomy in a visit to Byurakan Observatory (author’s photo).

  Ambartsumian also told me the history of Armenia. Armenians are very proud of their culture and history. Armenia is located very close to the middle of the fertile crescent — the region of the development of the earliest human civilizations. The Armenian civilisation is one of the oldest ancient civilisations which is still preserved today. The Armenian state was formed more than 3 thousand years ago. Armenia was the first sovereign nation to accept Christianity as a state religion in 301. The summer residence of the head of the Armenian Apostolic Church, the Catholicos of All Armenians, is located near the Byurakan Observatory in the Southern hill of the Aragaz Mountain. In my visits to Byurakan I often made long walks to the Catholicos residence. For many centuries in the middle ages Armenia was divided between the Ottoman and Russian Empires. It regained for a short period its independence in 1918, before it was permanently restored in 1991. The most tragic event in Armenian history was the Armenian Genocide during World War I. Together with my colleagues from the Byurakan Observatory I visited the Memorial of the Genocide.

  Business in Central Asia was rather bumbling, however, and when communicating with the outside world got easier at the end of the 1980’s, we started to investigate whether we could place the telescope on the Canary Islands instead, which already hosted several telescopes stationed there by Western European countries. These plans had to be rescinded when the Soviet Union was abolished and Estonia became independent. We could not find the necessary funds for the construction of the telescope under the new circumstances — Estonia did not have the means and we were unable to obtain such a large sum from elsewhere. Thus we decided to cancel our efforts to build our own Southern telescope.

  The development of astronomy in recent years has shown that there was a real need for a telescope constructed for the purpose of studying cosmological problems. There is a project underway between the astronomers of the USA, Japan, Germany, United Kington, Korea and several other countries — the Sloan Digital Sky Survey (SDSS). However, it is far more complex than we had anticipated when starting our telescope project. The originally planned SDSS I survey and its first extension SDSS II are finished, the new extension SDSS III is now underway, and there are many countries participating. Such projects are only feasible for transnational cooperations. Finland, incidentally, is taking part in a joint venture, the Nordic Optical Telescope, the telescope itself being situated in La Palma, Canarias Island.

  As for small observatories akin to Tartu Observatory, the only option is to use observational data received from major joint observatories. If we have the means to process the observational data which has nowadays proven to be very largescale, we can compete with other observatories quite successfully. Contemporary personal computers already have enough power for highly advanced scientific tasks, while the prices have settled to levels that are also affordable in Estonian conditions.

  5.3.2 Studies of ancient astronomy

  In addition to the philosophical seminars, ancient astronomy studies and conferences also turned out to be very fruitful. They were organised by Heino Eelsalu, who had a keen interest in ancient astronomy and the history of astronomy. Papers in this field were mostly published in local journals, so Eelsalu learned all the local languages of the Baltic region, starting with Latvian, our southern neighbours language, and ending with all Scandinavian languages, in addition to the major languages English, German, French and Russian. He started several series of publication of this topic: “Rara Astronomica in Estonia”, “Archivalia Cosmographica” etc. as Teated (Communications) of Tartu Observatory.

  Eelsalu found that rock art in Karelia and ancient Estonian folk songs regilaul contain important astronomical and historical information. So he initiated the formation of an Estonian Society of Prehistoric Art in the 1980’s. The Society made expeditions to Karelia, to study rock art from Lake Onega that date to 4,000–2,000 B.C. Many of our astronomers participated in these expeditions, including my daughter Maret. In these studies we had a very close collaboration with one of the best specialists of Estonian ancient culture, Mikk Sarv. Mikk writes on the meaning of regilaul as follows.1

  “The term ‘regilaul’ seems to derive from the word regi (sleigh) that denotes the oldest means of transport. A sleigh would take one across a boggy patch of land, even in summertime, when a carriage would be hopelessly stuck. In the winter, a sleigh would carry you straight across swamps and lakes and along rivers and winter roads. Sleighs would help traders to cover thousands of kilometres on their journeys and to connect people from coast to coast. Sleighs leave behind them two trails runnin
g towards the horizon on the plain fields. These trails are always next to each other but they can never meet. A song would help to make long journeys on a sleigh shorter, keeping the travellers awake and lifting their spirits.

  Estonian regilaul is an ancient and powerful means of communication for a headstrong nation with the rest of the world. To use it one needs to embrace timelessness and to picture in one’s soul the two trails behind a sleigh heading towards the horizon. Next to each verse there is another one that takes you in the same direction in a slightly different manner. Each verse moves along its trail carried forward by words beginning with and containing similar sounds. Singers cast ancient myths, fairy tales and shaman journeys into the form of regilaul and handed them down from one generation to the next. At the same time, regilaul as a form of art is also a highly sophisticated means of expression. Skilled singers can transform ordinary speech into regilaul and perform it at any time. This used to be common practice during festive rituals, such as weddings, or during incantations and swearing-in ceremonies.

  A song festival with choirs from all corners of Estonia was held in Tartu in 1869 to celebrate the abolition of serfdom. Since then song festivals have been held every five years as a tribute to the idea of freedom. In 1989, one-fifth of the Estonian population gathered on the song festival grounds in Tallinn to sing their country free from occupation. The Estonian regilaul has had a central role in all these instances of communal singing. If we take the very literal meaning of the word ‘laulupidu’ (song festival) then it means “to hold a song” (laulu pidama): i.e. to stop all other activities so that people can devote themselves to the song. For us Estonians, singing can be used to change the world.”

  I had once a personal experience of a spontaneous singing of a ‘regilaul’ at our family gathering at my country-home Egeri in summer 1999. One of my relatives, the wife of the son of my brother Rein, was from the South-Estonian region, where the ‘regilaul’ tradition is still alive. During the family gathering she started to sing, and I was able to record the event (see the movie in the website which accompanies this book).

  Another small remark of my own experience with song festivals. I have participated in several festivals. I had a very special experience at the festival in 1969, which celebrated 100 years since the first festival. I was there with Maret who was at the time 10 years old. On the last day of the festival we were already rather tired and had a 200 km drive to return home, so we left the festival and listened to it on our car radio. When the official program was over the joint choir started to sing spontaneously “Land of my Fathers, Land That I Love” (Mu isamaa on minu arm). Then something extraordinary happened: the whole festival public, more than a hundred thousand people, stood up and joined the choir. The people had tears in their eyes. This was the first solemn but very powerful peaceful demonstration of our idea of freedom. This song became an unofficial anthem for the Estonians during the occupation years. It took more than twenty years before real freedom was achieved, but this was the beginning. For many Estonians, including me, this song is the true anthem due to its role during the Soviet period, and its solemn melody and lyrics.

  Eelsalu initiated a series of interdisciplinary conferences devoted to paleo- astronomy and related topics. One of these conferences was held at the Viljandi theater on April 21, 1983. I remember very well this conference. The theatre hall was heavily crowded by Tartu University students, schoolchildren from Tartu and Viljandi, and leaders of the Estonian culture. Scientific presentations were combined with recitation performances on ancient religion, accompanied by folk music, performed by Mikk Sarv and his ensemble. At that time there were practically no other events supportive of patriotic sentiments. The atmosphere was very similar to the song festivals in 1989, where Estonia people sang the country free.

  1http://www.estinst.ee/publications/estonianculture/I_MMIII/sarv.html

  Chapter 6

  The nature of dark matter

  By the end of the 1970’s most objections against the dark matter hypothesis had been rejected. In particular, luminous populations of galaxies were found to have lower mass-to-luminosity ratios than expected previously, thus the need of extra dark matter both in galaxies and clusters became even stronger. However, there remained three problems:

  • It was not clear how to explain the Big Bang nucleosynthesis constraint on the low density of matter, and the smoothness of the Hubble flow — the main argument in favour of the classical cosmological paradigm.

  • If the massive halo (corona) is neither stellar nor gaseous, of what stuff is it made of?

  • And a more general question: in Nature everything has its purpose. If 90% of matter is dark, then there must be a reason for its presence. What is the role of dark matter in the history of the Universe?

  First I shall discuss baryons as dark matter candidates.

  6.1 Baryonic dark matter

  6.1.1 Early discussions on the nature of dark matter

  The nature of dark matter has been discussed from the very beginning of the discovery of the possible presence of dark matter in clusters and galaxies. It is clear that initially only baryonic dark matter candidates were considered.

  Zwicky (1937) writes: “We must know how much dark matter is incorporated in nebulae in the form of cool and cold stars, macroscopic and microscopic solid bodies, and gases”. Oort (1940) investigated the structure of the galaxy NGC 3115 and found that on the periphery the local mass-to-luminosity ratio is very high, of the order of 250 in Solar units. He notes: “The spectrum of the nebula shows the characteristics of G-type dwarfs. Since M/L cannot be much larger than 1 for such stars, the remainder must mainly consist either of extremely faint dwarfs having an average mass to light of about 200, or else of interstellar gas or dust”.

  The study by Kahn & Woltjer (1959) of the dynamics of the Local Group of galaxies is devoted mainly to the discussion of the possible nature of the matter which makes the system dynamically stable. The authors consider stars as possible candidates of the invisible matter and find that this is unlikely. As a possible candidate of the intergalactic matter they suggest ionised hydrogen (and helium). They find that the temperature of the gas should be about 5 × 105 degrees. The cooling time for such gas is of the order of 1010 years, thus the whole system is stable.

  6.1.2 Stellar or gaseous dark coronae

  When I started the modelling of the Galaxy and M31, my basic goal was to represent in the model all known stellar populations as well as possible. Our previous experience had shown that the best function to represent the mass and light distribution of individual populations is the generalised exponential function with variable shape parameter N. Using photometric data it was relatively easy to find the shape parameter, as well as scaling parameters defining the effective radius and the total luminosity of the population. To find the mass distribution from these data, only one additional parameter was needed: the mass-to-luminosity ratio of the population. This was the critical step in the model construction. As described in the previous Chapters I used various methods to calibrate the M/L values of galactic populations.

  In my Athens talk in September 1972 I considered the possible nature of the corona, and found that neither stars nor neutral gas can be considered as possible candidates for the coronal matter, see Ch. 4. In the Caucasus Winter School in January 1974 the problem of the nature of the corona was at the center of discussions. I listed my arguments against the stellar nature of galactic coronae.

  Physical and kinematical properties of stellar populations depend almost continuously on the age of the population. The oldest have the lowest metallicity and M/L-ratio, and there is no place to put the new population into this sequence. The continuity of stellar populations of various age is reflected also in their kinematic characteristics, such as the velocity dispersion and galactocentric centroid velocity, expressed in the Strömberg diagram, see Fig. 6.1. Here open circles mark metal-poor populations, while dots mark populations with normal metal abundance. Populati
ons were chosen using various physical parameters — variables of different type, star clusters of various age and metallicity etc. We see that there exists an almost continuous sequence of populations. Populations in our galactic models are actually representatives of populations which have a certain range of ages and metallicities. Similar sequences exist between kinematical and physical parameters, such as colour, M/L-ratio etc.

  Fig. 6.1 The Strömberg diagram for populations. In the horizontal axis we show the heliocentric centroid velocity of the population in the direction of the Galactic rotation; in the vertical axis we plot the mean velocity dispersion Open circles are for metal-poor populations, dots for populations with normal metal abundance. The numbers give the birth-dates in 109 years starting from the formation of the oldest populations, assuming for the age of the Galaxy 1010 years (Einasto, 1974b).

  The dark population is almost spherical and thus non-rotating. It has a much larger radius than all known stellar populations. Thus, in order to be in equilibrium in the Galactic gravitational potential, its objects must have much higher velocity dispersion than all known stellar populations. For this reason in the Stromberg diagram the dark population is located far away from all known populations above the diagram seen in Fig. 6.1. The conventional halo population, consisting of old metal-poor stars, has the highest velocity dispersion. This population has the lowest M/L-ratio among known populations. In contrast, the new dark population must have a very high M/L-ratio.

  This simple comparison of properties of known populations with those of the dark corona shows that the kinematics, the spatial distribution, and physical properties of the dark population are completely different from properties of known populations, and that there exist no intermediate ones which could form a link between known populations and the dark corona.